A fundamental goal of automotive heating, ventilating, and air conditioning (HVAC) systems is to make vehicle occupants comfortable. To achieve this goal, it is important that the design of the control system that establishes cabin conditions takes into account the relationship between comfort and the variables that affect comfort. Human comfort is a complex reaction, involving physical, biological, and psychological responses to the given conditions. Because of this complexity, the engineer must consider many variables and their possible interaction in the design strategy of such a control system or controller.
In an attempt to measure and control the many variables that affect comfort, modern automotive HVAC systems have many sensors and control actuators. A typical system might have a temperature sensor inside the cabin, one measuring ambient temperature outside and others measuring various temperatures of the system internal workings. The occupant may have some input to the system via a set point or other adjustment. Additional sensors measuring sun heating load, humidity, etc. might be available to the system. The set of actuators might include a variable speed blower, some means for varying air temperature, ducting and doors to control the direction of air flow and the ratio of fresh to recirculated air.
The traditional control strategy of an HVAC system utilizes a linear control equation in which an arbitrary number whose scale is chosen more for microchip related numerics rather than for physical significance. This matter of scaling may seem to be a minor point, however, if temperature control calibrations are to be pre-calculated in a vehicle simulation, it is essential to give this equation physical meaning and tie comfort, work, and vehicle-specific variables together.
Thus, there exists a need for formulating temperature control equations in terms of physical quantities, such as work, temperature and air flow rate. Such equations will then have a connection with available Computer Aided Engineering (CAE) tools for initial development in a computer rather than on a vehicle test trip.